Sure, but once you've licensed the images under a certain CC license, you can't really go back. Yahoo will probably obey the user choice (as I think they should) but legally aren't under any obligation to do so.
"Flickr members who don't want their CC-BY or CC-BY-SA photos to be eligible for sale as part of Flickr Wall Art can change the license (e.g. CC-BY-NC, etc.) and the images will be removed from the selection." https://www.flickr.com/help/forum/en-us/72157649323144986/pa...
What's interesting is that CC licenses are irrevocable. So if a hypothetical GreatPhotoPrints.com bulk-downloaded CC-BY- and CC-BY-SA-licensed photos, they could continue to sell coffee mugs, t-shirts, posters, canvas prints, etc. -- even if the Flickr user changed the license later. (And of course if Yahoo ever became desperate for cash, they could reverse the current policy decision that currently permits opting out.)